Boring apparatus of this kind comprise a driven drill string having a drill head the form of which may vary widely. The drill string is usually mounted on a carriage which runs on rails and is connected to a linear drive and has a rotary and/or forward feed (thrust) drive by which the string can be set in rotation and pressed forward into the ground.
To enable directional boring to be performed, such apparatus has a drill head having an eccentricity: that is to say, the drill head is such that lateral steering forces occur which bring about travel along a curve but which can be overcome when boring straight ahead. To do this, during the straight-ahead boring the drill head having the eccentricity rotates about the axis of the string with constant angular velocity so that the effect of the eccentricity is lost. On going over to curve boring the part exhibiting the eccentricity, or the drill head, is kept stationary in a particular angular position for a certain length of time, and remains in this angular position until completion of the curved path or so long as the predetermined curved path is maintained. If the drill head departs from the predetermined curve of the path, a correction of the angular position is necessary until the predetermined path curve is regained.
The nature of the drill string, of the eccentricity and of the drill head varies widely from case to case. Thus published European patent application 0 247 767 discloses a drill head connected to a rotary/thrust string and having an oblique face which permits straight-ahead boring so long as the drill head rotates uniformly and--in the absence of rotation--curved boring by lateral displacement of the soil in front of the drill head.
The known methods and apparatus for directional boring are all based on the principle that the drill string rotates during the straight-ahead boring and accordingly the drill head describes an envelope which is larger--usually substantially larger--than the diameter of the string or of the drill head, while during the travel along a curve the drill does not rotate and the is driven forward only by thrust and/or impact.
When boring along a curve, however, the advance into the ground is problematical, since the drill head is then not performing any excavating work, but only displacement work. In the case of soft soils this is not a problem, but in the case of harder or even stony soils the work of displacement required is quite considerable. Hence as a result of the tool being stationary in general either high thrust forces or special excavation methods are necessary. High thrust forces require a string which is equal to the task, in particular one with adequate resistance to buckling, with a correspondingly high weight, and can only be manufactured with corresponding high machine effort. This also applies to the case when the thrust when boring along a curve is effected by means of a percussion device. In addition, high thrust forces require a correspondingly firm anchorage, which it is particularly difficult to provide in the case of boring apparatus placed above ground and attached obliquely.
On the other hand, however, the string must also not be too rigid, since boring along a curve is only possible if the string is sufficiently elastic to bend to the corresponding curvature.
A boring apparatus is also known from European specification 0 195 559 of which the angled drill head, fitted at the end of a rotatable string, is provided with a concentric nozzle from which a high pressure jet issues to loosen and excavate the soil located in front of the drill head. While this apparatus avoids the need for high driving or thrust power when boring along a curve, for this purpose it nevertheless requires very high pump power to produce the fluid stream which excavates the soil. In addition, the soil excavation by means of the fluid jet cannot be precisely controlled, so that an accidental washout can lead to the formation of a cavity larger than that required for the forward travel. Such cavities can very easily lead to an undesired directional deviation. Independently of this, the directional stability when boring along a curve also suffers from the soil surrounding the drill head becoming greatly softened by the excavation or flushing fluid.
Finally, the high fluid pressures required result in the introduction of correspondingly large quantities of fluid into the ground, which is undesirable on both environmental and economic grounds. For the pressure fluid is generally a bentonite/water suspension, part of which remains in the ground while the major part flows back along the drill string towards the launch pit, where the suspension has to be collected and disposed of.
A further point is that the bentonite in the suspension brings about considerable frictional wear both in the supply line to the drill head and in the nozzle and as it flows back along the drill string.